Communication systems, such as land mobile radio and cellular communications systems, are well known. Such systems typically include a plurality of radio communication units (e.g., vehicle-mounted mobiles or portable radios in a land mobile system and radio/telephones in a cellular system), one or more repeaters (usually located at a fixed repeater site) and other equipment used in the processing and monitoring of communications. The repeaters are typically connected to other fixed portions of the system (i.e., the infrastructure) via wire connections, whereas the repeaters communicate with communication units and/or other repeaters within the coverage area of their respective sites via wireless link(s).
As is well known, such communication systems often employ a combination of frequency division duplexing (FDD) and time division multiple access (TDMA). In FDD, different frequencies are used for communication from the repeaters to the communication units (called the downlink) and from the communication units to the repeaters (called the uplink). In TDMA, the uplink and downlink frequencies are divided into blocks of time called time slots. Generally, in most TDMA systems, the time slots are of fixed length. On the downlink, the repeaters transmit continuously with different time slots destined for different communication units. On the uplink, the communication units take turns transmitting in different time slots.
Generally, in most TDMA systems, the majority of slots on the uplink are assigned to the different communication units by the repeater. This is done so that two or more communication units do not transmit at the same time. The remaining uplink slots are designated as random access slots by the repeater. During random access slots any of the communication units may transmit. Because the random access slots are not assigned to a particular communication unit, multiple communication units may transmit at the same time resulting in a collision and causing the repeater not to receive either transmission. One of the uses for the random access slots is for the communication units to request assignment of uplink slots. In such case, the request for slot assignments usually requires only a small amount of data to be transferred. Using a full slot for assignment request will therefore result in wasted bandwidth. Because of this some existing TDMA systems divide the random access slots in time to form multiple subslots from each random access time slots. A communication unit requesting uplink slot assignment transmits the request in only one of the subslots. This increases the number of opportunities for communication units to send request and thereby reduces the chance of collisions with transmissions from other communication units.
There is shown in FIG. 1 an example of a TDMA uplink slot 100 that has been divided into two subslots 102, 104 for use as a random access slot. Each of the subslots 102, 104 comprises a synchronization section 110, a data section 112, and a guard band 116. The synchronization section 110 is used by the repeater receiving the transmission to time synchronize with the transmitting communication unit. The data section 112 contains the information for requesting assignment of the downlink slot or other data. The guard band 116 is necessary to account for the propagation time between the communication unit transmitting a subslot and the repeater. The guard band 116 is used to keep the transmissions of communication units in adjacent subslots 102, 104 from overlapping.
Wireless links with wider bandwidths are being used in today's TDMA communication systems. Because of the wider signal bandwidths, these communication systems are able to send more information per unit of time than the smaller bandwidth signals used in previous communication systems. This allows the data section 112 of the TDMA subslots to be shortened since it takes less time to send the same amount of information. However, the guard band 116 can not be shortened since it is dependent on the propagation time between the communication units and the repeater and is therefore not effected by the signal bandwidth. The synchronization section can also not be significantly shortened beyond a certain point without negatively effecting time synchronization performance. Hence, as the signal bandwidth is increased, there is a limit to how short the subslots can be made in time. Since wider bandwidth TDMA communication systems often have shorter length time slots than smaller bandwidth TDMA communication systems, having increased signal bandwidth can actually result in having fewer subslots in TDMA random access time slots. It would therefore be desirable to find another method for dividing a TDMA time slot into subslots so as to increase the number of subslots.
As mentioned previously, if more than one communication unit attempts to transmit in a random access subslot at the same time, a collision will result and the repeater may not receive either transmission. In such case, the communication units may retransmit in a future random access time slot. Commonly, when doing such retransmissions each communication unit waits a random length of time before retransmission so that their retransmissions do not collide a second time.
In communication systems with wider bandwidth signals, each of the random access time slots will be able to contain a larger number of subslots. Because of this, the number of TDMA time slots allocated for random access is likely to be reduced. Hence, the amount of time that the communication unit is required to wait after a collision to do retransmissions is increased resulting in a longer delay to obtain permission to transmit on uplink reserved slots. It would be desirable to find a method to decrease the number of collisions that occur in random access slots so as to reduce the frequency of these delays.
Therefore there is a need for a new method of dividing a random access time slot into subslots in wider bandwidth TDMA systems. Advantageously, the method should more efficiently use the TDMA time slot than simply dividing the time slot in time into subslots. Additionally, there is a need for a method of transmitting in random access subslots that reduces the need for doing retransmissions in future random access time slots. This invention is directed to addressing these needs.